The invention relates to a process for the preparation of zeolite from incineration ash such as flyash and other aluminosilicate-containing substances.
As zeolite, which is crystalline aluminosilicate, naturally occurring zeolites are known, such as mordenite and clinoptilolite. Synthetic zeolites such as A zeolite and Y zeolite prepared from sodium silicate and sodium aluminate are used as a catalyst, molecular sieve and the like.
Meanwhile, it is also known to prepare zeolite by heating a slurry of flyash and an aqueous solution of alkali at 60xc2x0 C., 70xc2x0 C., 80xc2x0 C., 90xc2x0 C. or 100xc2x0 C. for several hours. This zeolite is generally called artificial zeolite. The artificial zeolite has similar ion exchange ability and adsorption ability as natural zeolite and synthetic zeolite do. However, its composition, crystalline structure, and purity are not precisely controlled, compared to those of synthetic zeolite. Meanwhile, the artificial zeolite is cheaper and suitable to mass consumption applications, for example, a soil improvement material and a deodorant. However, it should be much cheaper for broader applications, and thus an improvement on the process for its preparation is required.
In a conventional method where a mixture of 2.2 liters of an aqueous solution of alkali per kg of flyash is heated to 100xc2x0 C., it takes 5 hours (Japanese Patent Application Laid-open 6-321525). The present inventor found that it takes 20 hours to achieve a conversion of 50%.
Moreover, the reaction mixture obtained after the reaction contains a lot of by-produced water glass besides zeolite, and, consequently, the reaction mixture is sticky, which requires a high cost for solid (zeolite)-liquid separation. Further, the separated zeolite is still sticky and requires a lot of water and time to wash with water.
The present invention provides a process for preparing high quality artificial zeolite, preferably, at a conversion to zeolite above 50% in a short reaction time, where solid-liquid separation is made easier or unnecessary by reducing the amount of by-produced water glass (soluble silicate), and wasteful consumption of sodium hydroxide, potassium hydroxide and the like is decreased.
Conventionally, a concentration of alkali in an aqueous phase of a reaction system decreases with the progress of the reaction. The present inventor has now found that when the decrease in the concentration of alkali is at least suppressed or, preferably, avoided by evaporating water to remove from the reaction system during the reaction, the by-production of water glass is suppressed to make the reaction mixture non-sticky and, therefore, solid-liquid separation is easier or most of the water in the reaction system can be evaporated by the end of the reaction. In any case, there is no difficulty inherent to the conventional method in removing liquid from the reaction mixture. Further, the supplied alkali is used more effectively for the reaction.
Further, although it has been considered that even if the reaction temperature is raised above 100xc2x0 C., there are not attained sufficient advantages, e.g., an increase in crystallinity to counterbalance the cost (Japanese Patent Application Laid-Open 6-321525, paragraph 0041), the present inventor has now found that in the process according to the present invention, a higher conversion can be achieved in a shorter time by applying a reaction temperature above 100xc2x0 C., preferably above about 125xc2x0 C., and the amount of alkali metal hydroxide such as sodium hydroxide is smaller, and therefore the amount of water to be used is smaller, which is advantageous in heat efficiency.
The present invention is a process for preparing artificial zeolite comprising the steps of:
(a) mixing x kilograms of incineration ash or any other aluminosilicate-containing substance with y liters of an aqueous solution of alkali metal hydroxide having a concentration of 1 N or higher in a proportion, y/x, of 0.1 to 1.5 (liter/kg) to prepare a slurry; and
(b) heating the slurry from step (a) to cause a zeolite-forming reaction wherein water is continuously or intermittently removed by evaporation so that a decrease in a concentration of alkali which would otherwise naturally occur with a progress of the reaction is at least suppressed to thereby facilitate the reaction.
Also, the present invention includes the process, wherein the heating takes place at a temperature of higher than 100xc2x0 C. and at most 350xc2x0 C. A preferred range is from at least about 100xc2x0 C. to about 350xc2x0 C., and preferably the reaction takes place in a closed system such as an autoclave.